THE GENETIC-BASIS OF XX-XY DIFFERENCES PRESENT BEFORE GONADAL SEX-DIFFERENTIATION IN THE MOUSE

There is now a substantial body of data showing that in eutherian mamm als (mouse, rat, cow and man) XY conceptuses are developmentally more advanced (and consequently larger) than XX conceptuses of equivalent g estational age. This developmental difference is already discernible i n the preimplantation period and it has been suggested that the more a dvanced development of XY embryos may be a consequence of the preimpla ntation expression of Y chromosomal genes such as Sry or Zfy. In the p resent paper sex-chromosomally variant mice were used to analyse the g enetic basis of XX-XY differences as manifest at 10.5 days post coitum . The results show that the XX-XY difference is due to a combination o f a Y chromosome effect and an effect of the difference in X chromosom e constitution (2X v 1X). The Y effect is not dependent on the presenc e of Sry. In the light of this and other studies, it is concluded that the Y chromosome of most mouse strains carries a factor which acceler ates preimplantation development and that the resulting developmental advantage is carried over into the postimplantation period. The retard ing effect of two X chromosomes is then superimposed on this Y effect subsequent to the blastocyst stage but prior to 9.5 days Post coitum.